feat: let an account set its chip colour and label

Two optional keys in an [account.<key>] stanza. color fills the chip,
label sets its text.

Both belong to the account rather than to [tagcolors] because an account
tag is a different taxonomy: which mailbox a thread arrived in, not what
state it is in.

label is display only. "account-privateemail-danilo.macri" is a lot of
row for one bit of information, but the notmuch tag is never renamed, so
existing queries and external tagging are unaffected. Unset falls back to
the account key, and an empty label is ignored rather than rendering a
blank chip.

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